The present invention relates to determination of optical properties, e.g. polarization dependent loss (PDL), polarization mode dispersion (PMD), differential group delay (DGD), insertion loss, return loss and/or chromatic dispersion (CD), of a device under test (DUT) in transmission and in reflection of an optical beam.
Measurement setups for the above-mentioned purpose shall be as easy to handle as possible and shall reveal all optical properties of the DUT as fast as possible and with as little handling as possible. This means that the DUT should be fully characterized to all parameters required when it is once connected to the measurement setup. For a full characterization it is required to measure all parameters both in transmission and in reflection as fast as possible.
From the disclosure of work of Sandel et al (David Sandel, Reinhold Noxc3xa9, xe2x80x9cOptical Network Analyzer applied for Fiber Bragg Grating Characterizationxe2x80x9d, ECOC 97, 22-25 Sep. 1997, Conference Publication No. 448, (copyright) IEE, 1997, pp. 186-189; David Sandel et al, xe2x80x9cOptical Network Analysis and Longitudinal Structure Characterization of Fiber Bragg Gratingxe2x80x9d, Journal of Lightwave Technology, Vol. 16, No. 12, December 1998, pp. 2435-2442) it is known a method for polarization-resolved optical fiber Bragg grating characterization. However, in these disclosures only the reflection of the DUT is measured.
From a work of Froggatt at al (Froggatt et al, xe2x80x9cFull Complex Transmission and Reflection Characterization of a Bragg Grating in a Single Laser Sweepxe2x80x9d,) it is known to use a measurement setup to measure the group delay of a DUT in transmission and in reflection in both directions. However, with the disclosed measurement setup it is not possible to measure PMD or PDL. Moreover, the measurement setup disclosed in this article causes problems because the detectors used to detect the signals of reflection and transmission receive the signals of both directions simultaneously, i.e. the reflected signal of one direction is superimposed with the transmitted signal of the other direction and the transmitted signal of one direction is superimposed with the reflected signal of the other direction. Therefore, complex measures are necessary to distinguish between these signals without really knowing all impacts of this superposition of signals.
Therefore, it is an object of the invention to provide improved determination of optical properties of a DUT in one direction in transmission and in reflection of an optical beam.
The object is solved by the independent claims.
An advantage of the present invention is the provision of a fast way to convert a measurement setup of the above-mentioned art for measuring in transmission into a measurement setup which is able to measure the DUT in one direction in transmission and in reflection, simultaneously. In a preferred embodiment of the invention the inventive element comprises a semi-transparent mirror. This embodiment is easy to fabricate, easy to handle and cheap in production costs.
In a further preferred embodiment of the invention the element has a known proportion of transmission and reflection, more preferred also known optical properties, e.g. PDL, PMD, DGD, insertion loss, return loss, CD. It is preferred to have an element with substantially no PMD, DGD, insertion loss, return loss, PDL, and CD in the relevant wavelength range.
It is further preferred that the element is prepared in such a way that the optical properties can be adjusted. This embodiment guarantees more flexibility when using the inventive element.
In another preferred embodiment of the invention the element comprises a first beam splitter or coupler in an initial path of the beam for coupling out at least a part of the beam into a first path, an optical guide for guiding the part of the beam partly back into the initial path in reverse direction, the guide preferably comprising a second beam splitter or coupler in the first path for coupling the part of the beam back into the initial path. This embodiment realizes the invention without the necessity of using a semi-transparent mirror.
In another preferred embodiment of the invention the element comprises a first beam splitter or coupler in an initial part of the beam for coupling out at least part of the beam Into a first path, a mirror in the first path for reflecting back the part of the beam to the first beam splitter so that the first beam splitter partly guides the part back into the initial path in reverse direction and partly into a second path guiding the reflected signal in the initial direction.
Other preferred embodiments are shown by the dependent claims.
It is clear that the Invention can be partly or entirely embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit.